1. Field of the Invention
The present invention relates to a process for activating cells forming the elementary image points of an image display screen. It applies advantageously in cases in which the activation of the cells demands the provision of a current of short duration and high intensity. The invention also relates to an image display device which uses this process.
2. Discussion of the Background
The activation of the cells of a display screen demands the provision of a current whose intensity is all the higher the larger the number of cells to be activated simultaneously.
These conditions are found in various types of display screen to which the invention may therefore be applied, especially plasma panels, screens with light-emitting diodes, liquid crystal screens, or else screens of the type whose elementary cells use a so-called xe2x80x9cneedle effectxe2x80x9d phenomenon so as each to produce a beam of electrons. It should be noted that the simultaneity of actuation of the cells is more definite in screens which employ an effect termed the xe2x80x9cmemory effectxe2x80x9d.
Taking for example the screens of plasma panels in which the activation of the cells calls for a sizable current, and more particularly plasma panels (abbreviated to xe2x80x9cPAPsxe2x80x9d) of the ac type all of which employ the xe2x80x9cmemory effectxe2x80x9d, there are various types of ac PAPs: for example those which use just two crossed electrodes to define a cell, as described in French Patent FR 2,417,848; or again, ac PAPs of the so-called xe2x80x9ccoplanar sustainxe2x80x9d type, which are known in particular in respect of the European Patent document EP-A-0,135,382, and in which each cell is defined at the crossover of a pair of so-called xe2x80x9csustainxe2x80x9d electrodes, with one or more other electrodes used more particularly for addressing the cells.
The manner of operation of an ac PAP is explained below with reference to FIG. 1. To simplify the explanations, the diagram shown in FIG. 1 is that of a PAP with two crossed electrodes defining a cell.
The PAP comprises a network of electrodes Y1 to Y4 termed xe2x80x9crow electrodesxe2x80x9d, crossed with a second network of electrodes X1 to X4 termed column electrodes. To each intersection of row and column electrodes there corresponds a cell C1 to C16. These cells are thus arranged in rows L1 to L4 and columns.
In the example of FIG. 1, just 4 row electrodes Y1 to Y4 and 4 column electrodes X1 to X4 are represented, which define 16 cells C1 to C16 serving to form the display screen 1 of the PAP, but in practice, an ac PAP can comprise 1000 or more row electrodes and as many column electrodes, serving to define 1 million or more cells.
Each row electrode Y1 to Y4 is linked to an output stage SY1 to SY4 of a row control device 2, and each column electrode X1 to X4 is linked to an output stage SX1 to SX4 of a column control device 3. These two control devices 2, 3 are driven by an image management device 4.
The row control device 2 comprises a so-called xe2x80x9csustainxe2x80x9d generator 5 responsible for producing cell activation signals termed xe2x80x9csustain signalsxe2x80x9d SE. The sustain generator 5 delivers the sustain signals SE via an output circuit 6, which itself distributes them to each output stage SY1 to SY4 so that these signals SE are applied simultaneously to all the row electrodes Y1 to Y4.
It should be noted that a capacitance c PAP which symbolizes a so-called global capacitance exhibited by all PAPs has been depicted with dashed lines at the output of the sustain generator 5.
In a PAP, the elementary cell experiences just two states: the so-called xe2x80x9clitxe2x80x9d or xe2x80x9cwrittenxe2x80x9d state and the so-called xe2x80x9cunlitxe2x80x9d or xe2x80x9cerasedxe2x80x9d state. In the xe2x80x9clitxe2x80x9d state it can produce an electric discharge which itself produces light; in the so-called xe2x80x9cunlitxe2x80x9d state there is no discharge produced, and hence no light emitted. ac PAPs have in common that they benefit naturally, of not their technology, from the xe2x80x9cmemory effectxe2x80x9d mentioned above. The term xe2x80x9cmemory effectxe2x80x9d is understood to mean the effect which allows cells having two stable states to retain one or other of these states after the signal which triggered this state has disappeared.
In ac PAPs, the xe2x80x9cmemory effectxe2x80x9d is used with the aid of the sustain signals SE to activate the cells C1 to C16 which are in the xe2x80x9clitxe2x80x9d state, that is to say to bring about discharges and hence emissions of light in these cells, without modifying their xe2x80x9clitxe2x80x9d state or modifying the state of the cells which are in the xe2x80x9cunlitxe2x80x9d state.
It should be noted that the cells C1 to C16 are placed in the xe2x80x9clitxe2x80x9d state or in the xe2x80x9cunlitxe2x80x9d state as a function of the image which is to be produced, by addressing operations which are usually performed row by row. For this purpose, the row control device 2 generally comprises elements (not represented) which cooperate with the row output stages SY1 to SY4 so as, when a given row L1 to L4 is addressed, to superimpose addressing-specific signals on the sustain signals SE and to do so solely for the row electrode Y1 to Y4 which corresponds to the row L1 to L4 addressed.
FIG. 2a represents the sustain signals SE and FIG. 2b illustrates the phase relation between the inrush currents drawn by the row control device 2 and the sustain signals SE.
The sustain signals SE consist of voltage strobes following one another with a period P of the order of for example 8 to 10 microseconds.
These strobes are established on either side of a reference potential Vo which is for example earth. They vary between a negative potential V1, in which they exhibit a so-called negative plateau pxe2x88x92, and a positive potential V2 in which they exhibit an opposite plateau to the previous so-called positive plateau p+. These positive and negative potentials V2, V1 each have for example a value of 150 volts with respect to the reference potential Vo.
The reference potential Vo is applied to the column electrodes X1 to X4 in such a way that the sustain signals SE develop alternately positive and negative voltages, of 150 volts in the example, across the terminals of the cells C1 to C16, each of these voltages giving rise to a discharge in those cells which are in the xe2x80x9clitxe2x80x9d state.
These discharges in the cells C1 to C16. occur slightly after each negative or positive transition Tn, Tp of the voltage of the sustain signals, of the order for example of a few hundred nanoseconds after the establishing of the positive and negative plateau. To each of these discharges in the cells there corresponds an inrush current termed the xe2x80x9cdischarge currentxe2x80x9d ID which is provided by the row control device 2. In FIG. 2b it may be seen that the discharge current ID is in fact established after each start of a positive and negative plateau. Of course, the discharge current ID changes direction depending on whether it is established on the basis of a positive plateau p+ or a negative plateau pxe2x88x92.
The existence is also observed of another inrush current termed the xe2x80x9ccapacitive currentxe2x80x9d Ic which is in phase with each transition Tn, Tp of the sustain signals and which corresponds to the current required to charge, alternately positively and negatively, the overall capacitance c PAP exhibited by the PAP. This global capacitance of the PAP, of non-negligible value, is constituted by various stray capacitances and the like exhibited in particular by the screen 1 itself and which are formed for example by the row and column electrodes Y1 to Y4 and X1 to X4, the printed circuit tracks and the various connections and circuits, plus the stray capacitances exhibited by the elements responsible for deriving the sustain signals SE in the row control device 2. Thus, for example, the global capacitance c PAP can have a value of 10 nF in the case of a screen 1 having 4 or 5 dm2, possessing for example 512 row electrodes and 512 column electrodes which constitute 512xc3x97512 cells. Of course, the value of the global capacitance c PAP depends greatly on the technologies employed.
The discharge current ID corresponds to the sum of the currents consumed simultaneously by the discharges from all the cells which are in the xe2x80x9clitxe2x80x9d state. Its intensity can therefore vary considerably. The maximum intensity I1 of the discharge current ID, in the case of a screen having 512 row electrodes and 512 column electrodes, can attain a sizable value, of 10 amperes for example, which value itself also depends on the technologies employed.
The provision by the row control device 2 and more precisely by the sustain generator 5, of the sustain signals SE under a current whose intensity is as considerable as the maximum intensity I1, in a short time, poses problems which will be better understood with the aid of the explanations which follow regarding the manner of operation of the sustain generator 5 shown in FIG. 1.
The sustain generator 5 comprises a negative voltage source 7 and a positive voltage source 8, which respectively deliver the negative V1 and positive V2 voltages corresponding to the potentials of the negative and positive plateaux pxe2x88x92, p+ of the sustain signals SE.
The voltage sources 7, 8 are linked to a common point Pc, each by way of a switch element 10, 11. These switch elements consist for example of MOS type transistors, which make it possible to pass, in very short times, from a xe2x80x9cclosedxe2x80x9d or xe2x80x9conxe2x80x9d state in which they close the circuit, to an xe2x80x9copenxe2x80x9d or xe2x80x9coffxe2x80x9d state in which they open the circuit.
The switching elements 10, 11 are controlled from a clock device 13 by which they are turned xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d.
Thus, by turning xe2x80x9conxe2x80x9d the switching element 10 in series with the negative voltage source 7, the negative plateau pxe2x88x92 of the sustain signals SE is established at the common point PC; then by turning xe2x80x9conxe2x80x9d the switching element 11 placed in series with the positive voltage source 8, the positive plateau p+ of these sustain signals is established at the common point PC, the other switching element 10 having of course been turned xe2x80x9coffxe2x80x9d.
From the common point PC the sustain signals SE are transmitted to the output circuit 6 from where they are distributed to each of the output stages SY1 to SY4.
The discharges in the various cells C1 to C16 occur almost simultaneously, so that the discharge current ID is established and attains its maximum intensity Y1 in a very short time, of the order of 100 to 150 nanoseconds for example.
The voltage sources 7, 8 do not manage to deliver, with the required qualities, voltages V1, V2 nor the discharge current ID under which these voltages are delivered. This is due in particular to the internal resistances of the voltage sources 7, 8, which internal resistances are far from being negligible even for particularly sophisticated voltage sources, as is the case for those which are commonly used to fulfil the functions of the sources 7, 8.
The detrimental effects which result from this are for example:
sizable voltage drops and internal dissipations;
sizable time constants for the responses to the inrush currents;
relatively sizable variations in the voltage values V1, V2 as a function of the value of the discharge current ID.
These drawbacks are added to the high cost of the sources 7, 8 which must be used.
In addition to the limitations introduced by the voltage sources 7, 8, there are also limitations due to the switch elements 10, 11. This is because the whole of the discharge current ID passes alternately through one or other of these two switches 10, 11. These switches 10, 11 themselves also exhibit a non-negligible internal resistance (when they are in the xe2x80x9cclosedxe2x80x9d state), which provokes large voltage drops at their terminals. These voltage drops are all the more detrimental since their value varies with the variations in the intensity of the discharge current ID.
Under these conditions, and having regard to the various existing capacitances, the sustain generator 5 cannot always deliver the sustain signals under a current established in a fairly short time so as not to impair the physical phenomenon of the discharge in the cells.
These various limitations give rise to defects in the image displayed, such as in particular a variation in luminance as a function of the contents of the image, or else exaggerations or even reversals of the disparities in luminance between various regions of the image.
With a view to remedying these defects, a known solution consists in augmenting or in overdimensioning all or some of the elements which participate in order to derive the sustain signals SE and apply them to the cells, as well as in choosing and selecting the components. However, this solution greatly increases the costs while affording only partial improvements.
One of the aims of the present invention is to reduce or even eliminate the defects in the feeding of voltage and current to the cells, and more particularly the defects related to the shortcomings of the generator which produces the cell activation signals, that is to say the sustain signals in the case of a PAP. To this end, the invention proposes to power the cells with the aid of a solenoid, so as to produce a current source which is more suitable than the conventional sustain generators for providing currents of very short duration and very high intensity.
The invention relates to a process for activating the cells of an image display screen, consisting in cyclically producing signals termed xe2x80x9cactivationxe2x80x9d signals and in applying them to the cells, the activation signals having a period during which they engender at least one phase of activation of the cells, the activation of the cells determining a consumption of a current termed the xe2x80x9cdischargexe2x80x9dcurrent, the process being characterized in that in order to produce the activation signals, it consists in tapping off at the terminals of a solenoid, signals resulting from the application of at least one voltage to the solenoid, and in that it consists in causing a so-called xe2x80x9cmainxe2x80x9d current to increase and decrease in the said solenoid, of which current at least a part, in the course of the decreasing, constitutes the discharge current.
The invention also relates to an image display device comprising a screen having a plurality of cells and exhibiting a so-called global capacitance, a control device delivering activation signals whose application to the cells produces, cyclically, an activation thereof, the activation of the cells engendering the consumption of a current termed the discharge current, characterized in that the control device comprises a solenoid cooperating with switching means and at least one voltage source so as on the one hand, to produce at the terminals of the solenoid, signals serving to constitute the activation signals, and on the other hand so as to cause a current termed the main current which, at some time in its decrease, serves to constitute the discharge current, to increase and decrease in the solenoid.